Verc, I used to teach dynamic systems.
You are right about weight transfer, although I like the term load transfer better.
The theory I was referring to was the explanation of LFB oversteer by weight transfer, proposed by some of the earlier posts. Good thread.

 

Haha... Yeah- I'm kinda hangin by a thread. Actually, two of my chemistry professors did the same thing as you- started with physics, but switched to chemistry (the math was too much for them) This term, I'm taking Modern Physics (spec. relativity, quant. mech.), Analytical Mecahnics, and Analog Electronics. All apply to the physics major, but I find myself most interested in thre electronics class (figures, right?) The further I get into the math and numbers-physics behind things, the less interesting it has become. I took the general biology sequence my freshman year, but I figured I'd make more use of my natural math ability. Environmental sciences is what most interests me- but I also realize that this topic involves physics, and a bachelors degree in the field could still have me on the right track.

If I do continue with physics, I wont be working at a lab or a five-mile particle accelerator. If not environment-related, I'd most like to teach in a highschool, or possibly comm. college. I really think teachers are the people that make the most difference- because I think more young people need to become aware and interested in science in this modern world.

Physics bite to chew on: Did you know that time slows down when traveling at close to the speed of light? So, if you were to send someone the same age as you on a rocket at close to the speed of light on a round trip to Pluto, they would come back younger than you. Think of the possibilities!

Edit: sorry- I've gotten off the topic of the thread- has your original question been answered?

 

Physics major here. And I was one of the instructors at the rallyschool.

Basically, it's ALL about weight transfer. The point of LFBing is to quickly shift weight to the front of the car. You can achieve the same effect as LFBing by backing off the throttle suddenly. Doing it that way is called "trailing throttle oversteer". However, LFBing has some distinct advantages, the main point being that it's nearly instantaneous since the brakes are working directly on the wheels.

Even though you're braking and throttling at the same time, there's a lot of slack in the drivetrain, so your pulses on the brakes get absorbed by that and don't (or shouldn't) slow your engine down - allowing you to keep your turbo spooled up, or your NA engine in its powerband.

That slack in the drivetrain, couple with slack in the throttle cable connection and various linkages, plus whatever time it takes for your fuel delivery system to adjust to your changes, all means that there's a relatively long lag by doing it with the throttle.

The issue with FWD vs RWD is that almost every FWD car tends to understeer in turns, where as only a very few RWD ones do, and in general, LFBing is a cure for understeer. Even though they're AWD, Subarus respond well to LFBing because of two reasons: 1) they come from the factory WAAAAY understeering (for safety), and 2) they have a huge amount of weight on big ol' gooshy suspensions, so you can roll a lot of weight onto that outside front tire.


Trail-braking is kind of a different beast altogether, and can be done with either foot. Ordinarily you do all your braking in a straight line, then you stop braking and start turning, and, once you're past the steepest part of your turn, you start unwinding the steering wheel and rolling on the throttle. With trail braking, you start braking a little later, and hold the braking through the first part of the turn. The advantage is similar to above: you still have weight on the front part of the car when you start your turn, so an understeering car will understeer less. However, there then comes a time when you have to get off the brakes and back onto the throttle, and unless you're REALLY smooth (like, years worth of practice smooth), all that turning and rolling and nosing down and then nosing up again gets the car unsettled, and an unsettled car is an unhappy car.

 

this thread is awesome.

And the moral of the story is, major in physics and you will be good at rally driving...or get killed by a particle accelerator

 

Hoche, sorry if you took offense to what I said about the instructors- probably a generalization, and a flase one at that...

Do any of you know of any driving/rally schools in Northern California? And I mean, really north, cause I'm in Oregon, near the border. I've been wanting to look into enrolling- but I can't find any schools in my area.

 

Nah. It was sorta funny. I'd have to agree with you: there're a lot of really great drivers who don't really pay much attention to the physics and either do it all by feel or by following a few basic rules. And I can't count the number times I've gotten into arguments with people who claim their big brakes make them stop them faster.

Anyway, no, I don't know of any rally driving schools in NorCal or Oregon. I've been in discussions with a variety of people about doing a more driver-intensive version of the school. See the other thread.

 

So for LFBing your supposed to just tap the brakes, not actually apply them, which causes weight transfer (as the first people were saying), but then still be on the gas so your essentially just barely doing any weight transfer (since your only braking the same amount as there is drivetrain slack???) I'm not quite sure I understand, is the idea that in that instant when you slightly tap the brakes, weight shifts enough to start the car oversteering, and after it starts and your back on the gas, it'll continue to oversteer??? - Or do you keep pulsing to shift the weight back and forth, and when its forward, you oversteer, but then start understeering, then oversteer, etc (this one doesn't really make sense to me)???

I was under the impression that LFBing you kept pretty steady pressure on the gas, and applied just a little bit of brake, more or less steadily and it would oversteer. -- I feel kinda stupid, cuz on sunday, I was getting LFB to induce oversteer a little bit, but I'm not really sure what I was doing. I know I stayed on the gas, but I can't quite remember what I was doing with the brake

Wow, so many different (although some are very minor differences) opinions/explanations/ideas. I don't really know what to make of it all. Thanks everyone though, I feel like I'm learning a lot, and its great having so many people interested in helping out.

So far the things I've learnt for sure from this thread are;
a) particle accelerators are dangerous.
b) physics is an awesome major, so I should switch to electrical engineering, and
c)rallying is awesome.

Thanks all!!! :thumbsup:

 

You give it a shot (or multiple shots) of brake, not riding them.

If you do it hard enough and the surface is loose enough, you'll get enough sudden forward weight transfer that you can even get the back end to break loose and start rotating. Note that some (VW's, certainly) FWD cars have a brake balancer hooked up such that as the car dives, pressure is cut off to the rear brakes. You can still use LFB to get them to break, but it's just a little less responsive.

Anyway, once the back end's rotating, you have to make a decision. If the back end's rotating fast enough to maintain rotational momentum, you get off the brakes and just let the rear slide. Eventually (in FWD) the fronts will pull you out of it. If it's not coming out enough, you can floor the throttle and hold the brakes on a bit. The throttle will counteract the braking in the front, and hopefully you'll have the bias set so that the rears will stay locked, and you can flick the steering in to get the back end out. Of course, sliding the rear in a FWD car just slows you down, but since the whole point is that you've overshot the turn and are trying to compensate for that without dropping out of your powerband, this is good.

In an AWD car, the second sort of sliding is harder to do and plays hell with your diffs. In my OBS, I pretty much use it to help me bring the car in on a turn I've over shot, and if I need to lock the back end, I use the e-brake.

RWD just depends. If you're understeering and the car's in line, you can use it to pitch the back end out, but don't really get any benefits from riding them at all.

And that reminds me...I haven't been over to LeMans Karting in awhile, but it used to be that one of the fastest ways through turn one there was to simultaneously turn right and pop the brake pedal. You'd get this cool "BAM!" as the rears broke loose, the kart would rotate with them still spinning, and then they'd hook up again just in time to miss the berm around the corner on your left. Of course, those karts only have brakes in the rear, so you could do all that without losing any grip in the front...

 

Oh man - I feel a "what is your profession/what did you study in school" poll coming on...
Are we all geeks here?
For the record, physics bachelors, molecular biology/genetics masters.

MVWRX described it very well. Bottom line though, like any theoretical understanding in the end it all comes down to empirical testing to develop a feel for how it gets used. I always think of LFB like using a rudder to steer. Develop control in feathering the right and left feet on the controls and it's like you can steer with just your feet.
By contrast, e-braking lacks that finesse.
They're all just tools in the tool box. Just have to learn to apply them appropriately.

 

Last time I tried that at LeMans they black flagged me- telling me it against the rules to apply brake and throttle at the same time, but your right when you pull that move right you just rocket around the turns.

 

BTW what Hoche is saying is not inconsistent with the stuff I said. The "shots" of brake can definitely cause weight transfer.

 

i was doing it in gt3 haha it does work quite well... just feel so not right

edgar,

 

Here is my personal understanding.
I LFB daily on most of my turns to work and play.

LFB is to induce oversteering.
LFB is just jab onto the brake, not hold down the brake.
Like when you do a sudden hard stop on a straight, your head nod forward.
This is the purpose of LFB, to transfer the weight of the car from back to front.
(BTW, when the is on slope, downhill and uphill is different. This is in the case of flat road. Curves okay. Downhill == accelarate. Uphill == dec)

Right Foot on Gas is to keep the engine torque/power band(look at your dyno)
at the platu range where it is max.

For stock cars:
FWD tends to understeer.
RWD tends to oversteer.
AWD tends to understeer on entering the turn, normalize(with differential lag)
inside the turn, oversteer existing the turn.

Reducing speed(by braking or gas) reduce understeering/oversteering in action.

On contrary, hard accelateration induce more of above action.

Without the ALK installed on the stock Subaru, it tends to understeer
when you accelarate into the turn.
With the ALK installed, the weight distribution is now closer to 52/48
and LFB effect is not as obvious.

-Kendo.

 

lol, its beautiful man *tears of joy* .... but youre not getting my bud light....
from odyss3y @ subydudes

 

I use left foot braking to "brake boost" to build boost while racing from a roll.